Hebmann espig anj



inwater. After prises dippingand moved about th agitating the stones therein Patented May 26, 1931 UNITED STATES PATENT OFFICE and in Germany February 11,

surface about 5 minutes, removing them from the treatment of synthetic precious stones, parmelt and allowing them to"cool.. p class of In testimony whereof, we aifii; our signacorunds and spinels and consistingeither tures. entirely of pure alumina or of alumina con- HERMANN ESPIG. taining a small percentage of magnesia. An WOLFGANG TEUBN ER. object of the present invention is to provide a producing a perfectly smooth surface of these stones, such,as was hitherto only obtainable by grinding and polishing. Y

According to the present invention a perfectly smooth surface is obtained by treat ing the stones for a :few minutes with fused anhydrous sodium tetraborate (Na B O Preferably the moderately preheated stones are dippe into a melt of sodium tetraborate having a temperature of about 750 to 800 C. erein fora period of, say; 5 mlnutes. The stones are then taken out of the melt and allowed to cool, whereupon any sodium tetraborate still adhering to the surface of the stones is removed by rinsing them this treatment the surface of the stones has become perfectly smooth according to the present inventlon is equally applicable to cut stones such as are employed as bearings in watches and electrical app liances,'and to raw stones or shapes made of these stones.

e claim:

1.. The process which comprises treating the surface of synthetic precious stones, belonging to the class ofv corunds and spinels and being' mainly composed of alumina, with fused anhydrous sodium tetraborate.

2. The process of treating the surface of synthetic precious stones, belonging to the class of corunds and spinels and being mainly composed of alumina, which processcomthe moderately preheated stones into a melt of sodium tetraborate hav-" ing a temperature of about 750 to 800 C.

for a period 0 v e process 

